Native ants disperse invasive thistle seeds: Myrmecochory and the establishment of Carduus nutans and Cirsium arvense in Colorado

Many plant species are myrmecochores: they produce seeds with nutrient-rich appendages (elaiosomes) that attract ants for seed dispersal. Ants remove and consume the elaiosome, but leave the rest of the seed intact for germination. Because myrmecochorous relationships are not highly specialized, exotic plants may co-opt resident ants to provide dispersal services in a newly invaded range. We conducted seed-removal experiments, behavioral trials, and pitfall-trapping to determine whether native (or naturalized) ants disperse elaiosome-bearing seeds of Cirsium arvense and Carduus nutans, two invasive thistles introduced from Europe to the United States. Ants removed on average 15.0% of available Ci. arvense seeds and 10.9% of Ca. nutans seeds, suggesting they serve a moderate role in seed dispersal. At 3 of 6 study sites, ants removed more seeds than did vertebrate seed predators, which could facilitate the escape of these invasive seeds from predation. In behavioral trials, seeds of both plant species were presented to two native and one naturalized ant species, and the ants’ attraction to and ability to remove seeds was scored. These trials indicate that T. caespitum, which is naturalized in the United States, is likely an important disperser of Ci. arvense seeds, while D. insanus, a native ant, is the best disperser of Ca. nutans. A native seed harvesting ant, Pogonomyrmex occidentalis, ignored seeds of both plant species. Pitfall-trap data describing the relative frequencies of ant species at each study location will further describe which species are likely to be important dispersers of these two invasive plants.